The present invention relates, in general, to hybrid circuit assemblies, and more particularly, to a novel linear amplifier circuit assembly that provides a wide bandwidth and a high operating frequency.
Previously, the cable television (CATV) industry had used hybrid circuit manufacturing techniques to build low cost cascode amplifiers that provide linear amplification of CATV's high frequency signals. Among other things, the previous amplifiers generally were required to have a low cost, a wide bandwidth, and a gain that was flat within several tenths of a decibel (db). Generally, the amplifier's gain rolled off rapidly at frequencies above 600 MHz. The recent increase in cable television channels from 77 to 151, and the development of high power direct satellite transmission to the home has increased the need for linear amplifier assemblies having a maximum operating frequency above the 600 megahertz (MHz) limitation of previous linear amplifier assemblies. A typical amplifier is disclosed in U.S. Pat. No. 3,895,306 issued to Paul L. Rebels on Jul. 15, 1975 which is hereby incorporated herein by reference.
Elements of the cascode amplifier circuit were typically mounted on an aluminum oxide filled ceramic (alumina) substrate. In most cases, each semiconductor die of the amplifier was attached to a heat spreader that was on the alumina substrate. Each heat spreader typically was a small platform of high thermal conductivity material such as metal that was mounted between a semiconductor die and the alumina substrate. Each heat spreader absorbed a portion of the heat generated by a semiconductor die to assist in dissipating the heat. Wire bonds were used to provide electrical connection between each semiconductor die and metal interconnect patterns that were on the alumina substrate. Because the bonding wires between each semiconductor die and the interconnecting patterns on the alumina substrate had to pass over the die's heat spreader, the bonding wires were very long. The unusually long bonding wires had a high parasitic inductance that made the amplifier unstable at high frequencies and limited the amplifier's bandwidth.
Another parasitic element was added by the heat spreader itself. Since the heat spreader was a conductor that contacted the back of the semiconductor die, the heat spreader formed a parasitic collector to ground capacitance, thereby adding to the amplifier's bandwidth reduction and instability.
Accordingly, it is desirable to have a high frequency linear amplifier assembly that has a low cost, that has low parasitic inductance, that has low parasitic capacitance, that has high thermal conductivity, and that has a gain which is substantially flat up to frequencies in excess of 600 MHz.